Aircraft assembly includes fitting together many complex and often large components in precise relationships. For example, wings and tail planes (e.g., vertical stabilizer, horizontal stabilizer) may be assembled substantially whole and then attached to the corresponding section of the aircraft body. As another example, the fuselage of an aircraft may be the combination of several body assemblies where each body assembly is a barrel section of the fuselage. Some components, including some very large components such as the wings, fuselage sections, and tail assemblies, are designed to be assembled with high accuracy, e.g., with relative positional tolerances between parts of less than 0.005 inch (about 0.1 millimeters).
Shims are used extensively in the aircraft industry to fit and join together large components. Shims, also called fillers, are used to fill gaps (or voids) between joined parts. Gaps may be designed gaps that are designed to allow for manufacturing tolerance, alignment of components, and proper aerodynamic assembly of the aircraft. Shims may be individually sized to fit the corresponding gap with high accuracy (e.g., with a residual gap of less than 0.005 inch (about 0.1 millimeters)). The use of shims to fill gaps between mating parts results in more accurately assembled and more structurally sound aircraft.
Typically, each shim for a gap is custom fit to the individual parts being assembled. Design and fabrication of unique shims for each aircraft can be a time consuming and labor intensive process. Shim design and installation may be time consuming enough to significantly impact the speed of aircraft assembly.
One method to determine the proper shim size involves bringing the parts together in a test fit, identifying each gap, and measuring each gap to determine the dimensions and shapes of the custom shims. The sizes and shapes of gaps are probed by feeler gauges. Use of manual feeler gauges involves a progressive trial-and-error technique while use of electronic feeler gauges may be more automatic, yet still require placing the gauge in each gap to be measured. The feeler gauge approach is time consuming and tedious, and its accuracy may be dependent on the skill and experience of the person making the measurements. Incorrect measurements may result in ill-fitting shims which would lead to repeated measurements and production of shims until the proper fit is achieved.
Another method of fitting the gaps between mating parts, sometimes referred to as predictive shimming, involves scanning the interfacing part surfaces in an attempt to predict the exact shape of the gap between these surfaces. The parts are virtually fitted together based upon the engineering design. Shims are fabricated based on the virtual fit. A drawback of this approach is that the parts, especially large assemblies, may not fit together in the manner predicted virtually and, hence, the predicted shim shapes would be inappropriate. The parts may not fit together as expected because of deviations from the engineering design (even when the deviations are within tolerance), inaccurate relative location of the parts, and/or inaccurate surface measurements. In particular, this method relies on a high global accuracy of measurement and assembly, a feat that is challenging with large part joining like joining the wing to the fuselage.